Spectroscopy and Electromagnetic Radiation: Understanding Interactions in Analytical Chemistry

1. INSTRUMENTAL ANALYSIS CHEM 4811 CHAPTER 2 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

2. CHAPTER 2 INTRODUCTION TO SPECTROSCOPY

3. DEFINITIONS Spectroscopy - The study of the interactions of electromagnetic radiation (radiant energy) and matter (molecules, atoms, or ions) Spectrometry - Quantitative measurement of the intensity of one or more wavelengths of radiant energy Spectrophotometry - The use of electromagnetic radiation to measure chemical concentrations (used for absorption measurements)

4. DEFINITIONS Spectrophotometer - Instrument used for absorption measurements Optical Spectrometer - Instrument that consists of prism or grating dispersion devise, slits, and a photoelectric detector Photometer - Instrument that uses a filter for wavelength selection instead of a dispersion device

5. ELECTROMAGNETIC RADIATION - Also known as radiant heat or radiant energy - One of the ways by which energy travels through space - Consists of perpendicular electric and magnetic fields that are also perpendicular to direction of propagation Examples heat energy in microwaves light from the sun X-ray radio waves

6. ELECTROMAGNETIC RADIATION Wavelength (m) 10-11 103 Radio frequency FM Shortwave AM Gamma rays Ultr- violet Infrared Microwaves Visible X rays Frequency (s-1) 104 1020 Visible Light: VIBGYOR Violet, Indigo, Blue, Green, Yellow, Orange, Red 400 – 750 nm - White light is a blend of all visible wavelengths - Can be separated using a prism

7. ELECTROMAGNETIC RADIATION λ1 node amplitude ν1 = 4 cycles/second λ2 peak ν2 = 8 cycles/second λ3 ν3 = 16 cycles/second trough one second

8. ELECTROMAGNETIC RADIATION Wavelength (λ) - Distance for a wave to go through a complete cycle (distance between two consecutive peaks or troughs in a wave) Frequency (ν) - The number of waves (cycles) passing a given point in space per second Cycle - Crest-to-crest or trough-to-trough Speed (c) - All waves travel at the speed of light in vacuum (3.00 x 108 m/s)

9. ELECTROMAGNETIC RADIATION Plane Polarized Light - Light wave propagating along only one axis (confined to one plane) Monochromatic Light - Light of only one wavelength Polychromatic Light - Consists of more than one wavelength (white light) Visible light - The small portion of electromagnetic radiation to which the human eye responds

10. ELECTROMAGNETIC RADIATION - Inverse relationship between wavelength and frequency λα 1/ν c = λ ν λ = wavelength (m) ν = frequency (cycles/second = 1/s = s-1 = hertz = Hz) c = speed of light (3.00 x 108 m/s)

11. ELECTROMAGNETIC RADIATION - Light appears to behave as waves and also considered as stream of particles (the dual nature of light) - Is sinusoidal in shape - Light is quantized Photons - Particles of light

12. ELECTROMAGNETIC RADIATION h = Planck’s constant (6.626 x 10-34 J-s) ν = frequency of the radiation λ = wavelength of the radiation E is proportional to ν and inversely proportional to λ

13. INTERACTIONS WITH MATTER - Takes place in many ways - Takes place over a wide range of radiant energies - Is not visible to the human eye - Light is absorbed or emitted - Follows well-ordered rules - Can be measured with suitable instruments

14. INTERACTIONS WITH MATTER - Atoms, molecules, and ions are in constant motion Solids - Atoms or molecules are arranged in a highly ordered array (crystals) or arranged randomly (amorphous) Liquids - Atoms or molecules are not as closely packed as in solids Gases - Atoms or molecules are widely separated from each other

15. INTERACTIONS WITH MATTER Molecules Many types of motion are involved - Rotation - Vibration - Translation (move from place to place) - These motions are affected when molecules interact with radiant energy - Molecules vibrate with greater energy amplitude when they absorb radiant energy

16. INTERACTIONS WITH MATTER Molecules - Bonding electrons move to higher energy levels when molecules interact with visible or UV light - Changes in motion or electron energy levels result in changes in energy of molecules Transition - Change in energy of molecules (vibrational transitions, rotational transitions, electronic transitions)

17. INTERACTIONS WITH MATTER Atoms or Ions - Move between energy levels or in space but cannot rotate or vibrate The type of interactions of materials with radiant energy are affected by - Physical state - Composition (chemical nature) - Arrangement of atoms or molecules

18. INTERACTIONS WITH MATTER Light striking a sample of matter may be - Absorbed by the sample - Transmitted through the sample - Reflected off the surface of the sample - Scattered by the sample - Samples can also emit light after absorption (luminescence) - Species (atoms, ions, or molecules) can exist in certain discrete states with specific energies

19. INTERACTIONS WITH MATTER Transmission - Light passes through matter without interaction Absorption - Matter absorbs light energy and moves to a higher energy state Emission - Matter releases energy and moves to a lower energy state Luminescence - Emission following excitation of molecules or atoms by absorption of electromagnetic radiation

20. INTERACTIONS WITH MATTER Ground State: The lowest energy state Excited state: higher energy state (usually short-lived) Excited state Energy Ground state Absorption Emission

21. INTERACTIONS WITH MATTER - Change in state requires the absorption or emission of energy - Matter can only absorb specific wavelengths or frequencies - These correspond to the exact differences in energy between the two states involved Absorption: Energy of species increases (ΔE is positive) Emission: Energy of species decreases (ΔE is negative)

22. INTERACTIONS WITH MATTER - Frequencies and the extent of absorption or emission of species are unique - Specific atoms or molecules absorb or emit specific frequencies - This is the basis of identification of species by spectroscopy Relative energy of transition in a molecule Rotational < vibrational < electronic - The are many associated rotational and vibrational sublevels for any electronic state (absorption occurs in closely spaced range of wavelenghts)

23. INTERACTIONS WITH MATTER Absorption Spectrum - A graph of intensity of light absorbed versus frequency or wavelength - Emission spectrum is obtained when molecules emit energy by returning to the ground state after excitation Excitation may include - Absorption of radiant energy - Transfer of energy due to collisions between atoms or molecules - Addition of thermal energy - Addition of energy from electrical charges

24. ATOMS AND ATOMIC SPECTROSCOPY - The electronic state of atoms are quantized - Elements have unique atomic numbers (numbers of protons and electrons) - Electrons in orbitals are associated with various energy levels - An atom absorbs energy of specific magnitude and a valence electron moves to the excited state - The electron returns spontaneously to the ground state and emits energy

25. ATOMS AND ATOMIC SPECTROSCOPY - Emitted energy is equivalent to the absorbed energy (ΔE) - Each atom has a unique set of permitted electronic energy levels (due to unique electronic structure) - The wavelength of light absorbed or emitted are characteristic of a specific element - The absorption wavelength range is narrow due to the absence of rotational and vibrational energies - The wavelength range falls within the ultraviolet and visible regions of the spectrum (UV-VIS)

26. ATOMS AND ATOMIC SPECTROSCOPY - Wavelengths of absorption or emission are used for qualitative identification of elements in a sample - The intensity of light absorbed or emitted at a given wavelength is used for the quantitative analysis Atomic Spectroscopy Methods - Absortion spectroscopy - Emission spectroscopy - Fluorescence spectroscopy - X-ray spectroscopy (makes use of core electrons)

27. MOLECULES AND MOLECULAR SPECTROSCOPY Molecular Processes Occurring in Each Region 10-11 103 Gamma rays Ultr- violet Radio frequency FM Shortwave AM X rays Infrared Microwaves Visible 1020 104 Electronic excitation rotation vibration Bond breaking and ionization

28. MOLECULES AND MOLECULAR SPECTROSCOPY - Energy states are quantized Rotational Transitions - Molecules rotate in space and rotational energy is associated - Absorption of the correct energy causes transition to a higher energy rotational state - Molecules rotate faster in a higher energy rotational state - Rotational spectra are usually complex

29. MOLECULES AND MOLECULAR SPECTROSCOPY • Rotational Transitions • - Rotational energy of a molecule depends on shape, • angular velocity, and weight distribution • - Shape and weight distribution change with bond angle • - Molecules with more than two atoms have many possible shapes • - Change in shape is therefore restricted to diatomic molecules • - Associated energies are in the radio and microwave regions

30. MOLECULES AND MOLECULAR SPECTROSCOPY • Vibrational Transitions • - Atoms in a molecule can vibrate toward or away from each • other at different angles to each other • - Each vibration has characteristic energy associated with it • - Vibrational energy is associated with absorption in the • infrared (IR region) • Increase in rotational energy usually accompanies increase • in vibrational energy

31. MOLECULES AND MOLECULAR SPECTROSCOPY • Vibrational Transitions • - IR absorption corresponds to changes in both rotational and • vibrational energies in molecules • - IR absorption spectroscopy is used to deduce the structure • of molecules • - Used for both qualitative and quantitative analysis

32. MOLECULES AND MOLECULAR SPECTROSCOPY • Electronic Transitions • - Molecular orbitals are formed when atomic orbitals • combine to form molecules • - Absorption of the correct radiant energy causes an outer • electron to move to an excited state • - Excited electron spontaneously returns to the ground state • (relax) emitting UV or visible energy • - Excitation in molecules causes changes in the rotational • and vibrational energies

33. MOLECULES AND MOLECULAR SPECTROSCOPY • Electronic Transitions • - The total energy is the sum of all rotational, vibrational, and • electronic energy changes • - Associated with wide range of wavelengths • (called absorption band) • - UV-VIS absorption bands are simpler than IR spectra

34. MOLECULES AND MOLECULAR SPECTROSCOPY • Molecular Spectroscopy Methods • - Molecular absorption spectroscopy • - Molecular emission spectroscopy • - Nuclear Magnetic Resonance (NMR) • - UV-VIS • - IR • - MS • - Molecular Fluorescence Spectroscopy

35. ABSORPTION LAWS Radiant Power (P) - Energy per second per unit area of a beam of light - Decreases when light transmits through a sample (due to absorption of light by the sample) Intensity (I) - Power per unit solid angle - Light intensity decreases as light passes through an absorbing material

36. ABSORPTION LAWS Transmittance (T) - The fraction of incident light that passes through a sample 0 < T < 1 Io = light intensity striking a sample I = light intensity emerging from sample Io I

37. ABSORPTION LAWS Transmittance (T) - T is independent of Io - No light absorbed: I = Io and T = 1 - All light absorbed: I = 0 and T = 0 Percent Transmitance (%T) 0% < %T < 100%

38. ABSORPTION LAWS Absorbance (A) - No light absorbed: I = Io and A = 0 Percent Absorbance (%A) = 100 - %T - 1% light absorbed implies 99% light transmitted - Higher absorbance implies less light transmitted

39. ABSORPTION LAWS Beer’s Law A = abc A = absorbance a = absorptivity a = ε [molar absorptivity (M-1cm-1) if C is in units of M (mol/L)] b = pathlength or length of cell (cm) c = concentration

40. ABSORPTION LAWS Beer’s Law - I or T decreases exponentially with increasing pathlength - A increases linearly with increasing pathlength - A increases linearly with increasing concentration - More intense color implies greater absorbance - Basis of quantitative measurements (UV-VIS, IR, AAS etc.)

41. ABSORPTION LAWS Absorption Spectrum of 0.10 mM Ru(bpy)32+ λmax = 452 nm

42. ABSORPTION LAWS Absorption Spectrum of 3.0 mM Cr3+ complex λmax = 540 nm

43. ABSORPTION LAWS Maximum Response (λmax) - Wavelength at which the highest absorbance is observed for a given concentration - Gives the greatest sensitivity

44. ABSORPTION LAWS Deviations from Beer’s Law - Deviations from linearity at high concentrations - Usually used for concentrations below 0.01 M - Deviations occur if sample scatters incident radiation - Error increases as A increases (law generally obeyed when A ≤ 1.0

45. CALIBRATION METHODS Calibration - The relationship between the measured signal (absorbance in this case) and known concentrations of analyte - Concentration of an unknown analyte can then be calculated using the established relationship and its measured signal

46. CALIBRATION METHODS Calibration with External Standards - Solutions containing known concentrations of analyte are called standard solutions - Standard solutions containing appropriate concentration range are carefully prepared and measured - Reagent blank is used for instrumental baseline - A plot of absorbance (y-axis) vs concentration (x-axis) is made

47. CALIBRATION METHODS Calibration with External Standards

48. CALIBRATION METHODS Calibration with External Standards - Equation of a straight line in the form y = mx + z is established m = slope = ab z = intercept on the absorbance axis - Concentration of unknown analyte should be within working range (do not extrapolate) - Must measure at least three replicates and report uncertainty

49. CALIBRATION METHODS Method of Standard Additions (MSA) - Known amounts of analyte are added directly to the unknown sample - The increase in signal due to the added analyte is used to establish the concentration of unknown - Relationship between signal and concentration of analyte must be linear - Analytes are added such that change in volume is negligible

50. CALIBRATION METHODS Method of Standard Additions (MSA) - Different concentrations of analyte are added to different aliquots of sample - Nothing is added to the first aliquot (untreated) - Concentrations in increments of 1.00 is usually used for simplicity - Plot of signal vs concentration of analyte is made